Von Neumann Model

The Von Neumann model is the most successful, and the dominant paradigm of computing for N decades.

 

It consists of following 5 components : 

1. Memory

2. Processing unit

3. Input / Output

4. Controul unit

 

And it looks like this :

The Von Neumann Model

 

1. Memory

Memory stores Programs and Data.

A simple Memory Array (4 locations X 3 bits)

 

Address space : Total number of uniquely indentifiable locations in memory

ex) 16-bit addresses -> the address space is 2^16

Addressability : How many bits are stored in each location

 

16-gigabyte memory means there are 2^34 distinct memory locations, and each stores 8 bits (=1 byte).

 

Memory Address Register (MAR) : Stores information of address itself

Memory Data Register (MAR) : Holding the contents of a memory location on its way to/from the storage

 

 

To write a value in a memory location, 

1. Write the address of the memory in the MAR, and the value to be stored in the MDR

2. Activate the Write Enable signal, and the value in MDR is written to address specified by the MAR.

 

To read a value from from a memory location

1. Load the MAR with the address we wish to read from.

2. Data in the corresponding location gets placed in the MDR.

 

 

2. Processing Unit

 

Processing Unit performs actual computations, and processing of information.

 

Arithmetic and Logic Unit (ALU)

Internal structure of  ALU

 

- Performs basic arithmetic functions and basic logic operations

- Implemented based on Combinational logic circuit.

- Processes data elements of a fixed size referred to as the "word length" of the computer. 

- Most microprocessors that are used in cellphones, PCs or workstations today have a word length of 64 bits or 32 bits. 

- There are more than 1 ALUs in a processing unit.

 

Temporary storage

- Small amount of storage very close to ALU.

- Exists in order to avoid the much longer access time.

- Temporary storage is usually a set of registers (faster than memory)

A 4-bit D-FF based register

 

3. Input / Output

- Input : Keyboard, Mouse, Scanner, Disks..

- Output : Monitor, Printer, Disks..

 

4. Control Unit

- Control Unit makes all the other parts of the computer play together; conductor of an orchestra, to say.

 

Instruction Register (IR) : Contains the instruction (keeps track of which instruction is being processed)

Program Counter (PC) or Instruction Pointer (IP) : Contains the next instruction

 

 

 

Two Key Properties of Von Neumann Model

1. Stored program

- Instructions are stored in a linear memory array

- Memory is unified between instructions and data

- The interpretation of a stored valued depends on the control signals

 

2. Sequential instruction processing

- One instruction processed at a time

- PC (IP) indentifies the current instruction, and advances squentially except for transfer instructions.